The present invention relates to waterborne coating compositions including, basecoating and clear coating compositions.
Over the past several years there has been a decided trend toward the reduction of atmospheric pollution caused by the volatile solvents which are emitted during painting processes. This trend has been particularly followed by the automotive industry where research efforts have been focused on the objection of fine quality, glamour-like automotive finishes while at the same time reducing or eliminating the organic solvents utilized in the coating compositions.
One approach to emissions control has been the use of water as a solvent in automotive coatings compositions so that organic solvents can be sharply reduced. U.S. Pat. Nos. 4,539,363 and 4,403,003 both to Backhouse, describe a major advance in this art; these patents teach that particulate dispersions (latices) can provide color coats in a basecoat-clearcoat application which are less sensitive to variations in humidity during application and more resistant to popping during baking.
A drawback to this technology si low application solids, presumably due to he water soluble thixotropy-promoting reaction of the aforesaid Backhouse compositions, which forces the applicator to spray large volumes or multiple coats of paint to achieve sufficient film thickness. A second drawback which is characteristic of acrylic latices is that significant organic solvent is still required for flow and coalescence. Thirdly, such products are not well adapted for application over flexible substrates, due to the non-elastomeric nature of crosslinked acrylic polymer films.
Another approach to solve the problems associated with waterborne coatings is the use of polyurethane polymers in aqueous media as described in U.S. Pat. Nos. 4,046,729 to Scriven et al. and 4,489,135 to Drexler et al. While these materials lead to coating switch outstanding flexibility and low organic solvent content, they tend to contain larger fractions of water soluble polymer which can give rise to the problems described in the Backhouse reference and also suffer for the problem of very low applications solids.
It is desirable, therefore, to have waterborne coating compositions which are capable of being formulated at high application solids, with low volatile organic content, and with reduced sensitivity to fluctuation in relative humidity.